This invention relates to an optical position input device of the type which includes means for preventing an error of detection from being caused by disturbant light.
An optical position input device is normally located in front of an image display device such as a CRT display or an LCD and is used to input coordinates to a computer. Such an optical position input device conventionally includes a large number of light emitting elements and a large number of light receiving elements arranged in an opposing paired relationship in directions of the X- and Y-axes of an outer periphery of a front surface of a screen of a CRT display or the like, and the light emitting elements and the light receiving elements are scanned to be successively selectively driven by a multiplexer so that when a light signal developed from a light emitting element is interrupted by a finger or the like during such scanning, the position at which such interruption of light occurs is detected to produce a corresponding position signal.
Such optical coordinate input devices are disclosed in U.S. Pat. Nos. 3,764,813, 3,775,560, 3,860,754 and 4,205,304, and optical coordinate input devices which include means for preventing an error of detection are disclosed in U.S. Pat. Nos. 4,243,879, 4,585,940 and 4,591,710.
By the way, if disturbant light is incident to a light receiving element, sometimes a light reception signal developed from the light receiving element may not be lowered below a predetermined value even if a light signal delivered from an opposing light emitting element is interrupted by a finger or the like. Thus, an error of detection is readily caused by disturbant light.
Technical means which eliminate an influence of disturbant light are disclosed in U.S. Pat. No. 4,243,879 and Japanese patent publication No. 61-25161.
In the technical means disclosed in Japanese patent publication No. 61-25161, just before a light emitting element and an opposing light receiving element are selectively driven, a light reception signal originating from incident disturbant light is developed from the light receiving element and is held in memory. Then, the light reception signal thus held in memory is subtracted from a light reception signal developed from the light receiving element when it is selectively driven in order to produce a signal indicative of whether or not the light signal has been interrupted.
According to the technical means mentioned just above, if only disturbant light is incident to a light receiving element just before the light receiving element is selectively driven, a light reception signal corresponding to disturbant light at a preceding point of time nearest to the point of time at which the light receiving element is selectively driven can be subtracted from a light reception signal developed from the light receiving element when the light receiving element is selectively driven, and consequently, correcting operation to eliminate an influence of disturbant light can substantially accurately follow such disturbant light which is varying in intensity.
By the way, in order to finely detect a position signal on a screen, the pitch of light emitting elements and light receiving elements to be arranged in directions of the X- and Y-axes must be fine. However, since light emitting elements have a wide directivity, a light signal delivered from a light emitting element tends to be received not only by an opposing light receiving element but also by light receiving elements which are located adjacent the opposing light receiving element and not opposed to the light emitting element. Therefore, the technical means disclosed in Japanese patent publication No. 61-25161 has a drawback that light receiving elements cannot be arranged by a pitch finer than the width of a zone irradiated by a light signal delivered from a light emitting element. To the contrary, if the directivity of light emitting elements is made sharper, the light emitting elements will be more expensive, and this will not present a practical solution to the drawback.